1. Field of the Invention
The present invention relates to a static electricity removal method and a static electricity removal apparatus for a copied image carrier (such as, print paper), which supplies reverse polarity ions to the back of the image carrier to eliminate the charged state of said image carrier, after the charged image carrier has accepted a transferred image.
2. Description of the Related Art
In a copier or a printer utilizing the principles of electrophotography, (1) light illuminates an evenly charged photosensitive body to form a latent image as a charged pattern, (2) a toner powder is sprinkled on the latent image to form a transferable image according to the charged pattern, and (3) an image carrier, such as print paper, is placed on the surface of photosensitive body to collect the toner image, in the process of placing the image carrier, such as the paper, on to the surface of photosensitive body to collect the toner image, (4) the image carrier is charged to a reverse polarity, compared to the toner image, for securing the attractive force between the image carrier and the toner image, (5) and after the image has been transferred, the reverse polarity ions on the image carrier are removed from the back face of the image carrier for simplifying the separation of image carrier from the photosensitive body.
For example, in a laser printer using a cylindrical photosensitive body, the processing units of the above mentioned items (1) through (5) are arranged in sequence on the circumference of the rotatable photosensitive body, and in one rotation of the photosensitive body turning at a high speed, the processing of aforesaid Items (1) through (5) are sequentially executed on the line crossing the cylindrical face (See FIG. 2).
That is to say, (1) a continuous laser beam scans the cylindrical face of the photosensitive body which is charged to +400 V to drop the charge at the illuminated point and to form a latent image by an electric charge pattern, (2) a black toner is sprinkled on the latent image to selectively hold it with electrostatic electricity on the latent image and to develop the charge pattern, (3) the developed black toner image is transferred to the surface of the image carrier (paper), (4) the paper, prior to its getting in touch with the photoelectric body, is irradiated with negative ions from its back side and is charged to -200 V, and (5) then, with the paper coming in contact with the photosensitive body, positive ions are irradiated from its back side to offset the electric charge on the paper.
Here, any ion generating unit which can be used in the applications of Items (4) and (5) comprises wire electrodes arranged opposite to the cylindrical face of photosensitive body. The wire electrodes are applied with a high voltage to perform a discharge for forming positive and negative ions in the air. The polarity of voltage being applied to the wire electrodes decides which of these positive and negative ions are utilized. In the application of Item (4), for example, a relatively high direct current voltage, such as -7 kV, is applied to a single wire electrode, whereas in the application of Item (5) an alternating current voltage of 12 kV, peak-to-peak, having a bias voltage of 1000 V (direct current) is applied to two wire electrodes arranged in parallel.
A high processing speed is required from a copier and a printer utilizing the principle of electrophotography. However, (5) the electricity removal unit for emitting a reverse polarity ion against the charged state of image carrier from the back face of image carrier becomes an obstruction to the realization of high speed.
In other words, if high speed processing is used the speed of the image carrier is increased and this shortens the time available to remove the change from the image carrier. Therefore, if the ion irradiation intensity is increased to complete the electricity removal within a short time, a part of toner adsorbed on the image carrier will be repelled by the image carrier and return to the photosensitive body, and the quality of a printed image or printed letters will be reduced.
However, unless the ion irradiation intensity is increased, the electrostatic charge cannot be completely removed from the image carrier, the photosensitive body cannot be easily separated from the image carrier, and the static electricity on the image carrier makes the handling of the image carrier difficult at each of the later processes.
For instance, in the foregoing laser printer, using a cylindrical photosensitive body as a photosensitive body, (4) the black toner image electrostatically holds on to the charge pattern (latent image) by a charge of +50 V on the cylindrical face of photosensitive body is overlapped with the paper charged to -200 V for electrostatically adsorbing the black toner image on the paper side. If the bias voltage to be impressed to the wire electrode should be increased up to 1500 V to increase the ion irradiation intensity (output current) and to rapidly offset the charged state of paper, the attractive force on the cylindrical face of photosensitive body against the black toner image becomes more influential than the attractive force of the paper. Therefore, when the paper has been separated from the photosensitive body, the black toner image may be held by the photosensitive body.
However, if the bias voltage to be impressed to the wire electrode should remain at 1000 V in order to leave the black toner image on the paper, the electric charge removal becomes insufficient when the photosensitive body turns at a high speed, and the paper cannot be easily separated from the photosensitive body. Therefore, a mechanical means for separation (a scraping blade, etc.) may be required. The paper jamming is likely to be caused also in the paper feeding mechanism at the later stage.